Energy harvesting refers to the use of ambient energy sources to locally generate electrical power. This power can then be used to supplement or replace battery or mains power supply especially in small-scale electronic applications. Vibration or motion is an example of such an ambient energy source that is an abundant source of energy that can be found in many different forms. A number of devices that can make use of vibration or motion have been proposed. The general principle underlying these implementations is to introduce an electrical damping mechanism, often referred to as the transducer, (e.g. an electromagnetic or piezoelectric transducer) counteracting the force exerted by a “proof” mass moving inside a frame. The frame needs only one attachment point to the host structure and thus the motion of the proof mass is caused by inertial reaction forces to the external acceleration, which can be linear or rotational. Consequently, these devices can be broadly categorised as inertial generators.
For continuous rotation, where there is no rotational acceleration, another method has been used. A device known as a gravitational generator relies on an eccentric proof mass and gravity to create a counteracting moment to the host rotation and thus extract power from it through a transducer.
Electromagnetic transducers are widely used for power generators of either conventional, inertial or gravitational type. They rely on the relative motion between two principal components. In general the first principal component, the stator, is held stationary by some sort of mechanical attachment to a base structure. The motion of the second principal component, called a rotor in the case of a rotational device, is electromagnetically coupled to the first principal component. Thus, displacement of the second component requires either a force (for linear implementations) or a torque (for rotational implementations) and by doing work against this force/torque electric power can be generated. The nature of this force or torque (as the case may be) is defined by the general nature of the device and can thus be inertial, gravitational or, in conventional structures, be provided by introducing a second attachment point that connects the second principal component to a moving structure. With a conventional arrangement, large transducer forces can be achieved between the two principal components and consequently the power output can be very high, which is ideal for power generation on a large scale. However, the possibility of having a second attachment point requires more space and is in fact not always easily achievable. One example, not exclusive of other possible applications, can be the installation of a pressure sensor inside a vehicle tyre, where attachment to the moving parts is conveniently possible but connection to a secondary, stationary structure is impeded. Another possibility is placement inside the human body, where attachment to two relatively moving parts/organs can result in highly intrusive surgery.
Mainly in linear devices, piezoelectric transducers have also been used. The key property of a piezoelectric material is that it responds to an applied stress by an accumulation of charge. The advantage is that high output voltages can be achieved without the need for any gearing and that the achievable energy is a function of the stresses inside the material and not the relative velocity of the two principal components as is the case for electromagnetic transducers. Good results have been accomplished using bending structures, because the stresses in a direction perpendicular to the direction of bending are high. Another way to increase the electromechanical coupling is to operate at the resonance frequency of the piezoelectric element. The difficulty of incorporating bending beams in rotational devices has made piezoelectric implementations of this type complicated to date.
A common disadvantage of linear devices is a strong dependence on the orientation of the device and the direction of excitation acceleration. In many cases, gravitational acceleration has to be overcome in order for the internal mass to be able to move at all. This problem can be bypassed if the orientation of the device is constant and the primary direction of excitation is other than (and preferably in a plane perpendicular to) gravity. This is however not always straightforwardly achievable and causes problems in strongly varying conditions, e.g. in human motion. A device with a proof mass rotatable around an axis, this axis being stationary in relation to the host, and a centre of mass not in line with this axis has the advantage of being operable under gravitational as well as inertial circumstances.
There is therefore a need to address this problem.